For more than a century, late blight on tomato and potato plants has been a cause of concern to plant breeders. Late blight is common in areas that have cool wet weather and it can be devastating during the rainy season when the humidity is high. Although late blight is a worldwide problem, it is particularly prevalent in the eastern United States, Mexico and the Far East.
Late blight is caused by a fungus called Phytophthora infestans ("P. infestans"). Phytophthora infestans can survive on plant debris, potato tubers, and also on Solanaceous weeds. Spores of this fungus are typically disseminated during windy, rainy periods; however, the fungus can be disseminated on infected plant debris and fruit. Upon landing on the leaves, stems and fruit of tomato plants, spores can rapidly infect the plant tissues causing tissue collapse and eventual death of the plant.
Presently, there have been two physiologic races described for P. infestans in tomato. The first is P. infestans race 0 which is pathogenic on commercial varieties that contain no resistance genes. The second is P. infestans race 1 which is pathogenic on commercial varieties that contain the Ph.sub.1, resistance gene.
In 1952, resistance to P. infestans was reported in a wild tomato relative, Lycopersicon pimpinellifolium. This resistance was found to be conferred by a single dominant gene (Ph.sub.1 ; formerly known as TR.sub.1) that gave resistance to the tomato strain of P. infestans race 0. However, the tomato strain of P. infestans race 1 overcomes the resistance conferred by Ph.sub.1. See Gallegly, M. E,. "Resistance to the Late Blight Fungus in Tomato," (1960) Campbell Plant Science Seminar p. 116. In addition, multigenic resistance was identified in the tomato line West Virginia 700 (PI204996). This resistance was not complete and conferred only a tolerance to P. infestans race 0 and 1. When the environmental conditions were favorable for P. infestans race 1 development, these tolerant plants sustained significant disease.
The cultivated tomato, Lycopersicon esculentum, is one of the most important vegetable crops in the United States and worldwide, with several million tons being produced each year in the United States alone. The commercial importance of the crop has necessitated a constant effort to improve cultivated varieties. Therefore, there is a need for cultivated tomato varieties that exhibit disease resistance to P. infestans race 1.
It is an object of the present invention to disclose a novel Lycopersicon pimpinellifolium cultivar that can be used as a source of resistance to the tomato strain of P. infestans race 1. It is a further object of this invention to provide Lycopersicon esculentum plants which are resistant to the tomato strain of P. infestans, races 0 and 1 and remain resistant to P. infestans race 1 in the field when the disease pressure is high.